This invention relates generally to optical signal processing systems and more particularly to beamsteering systems for phased array systems such as ultrasound devices and radar systems.
Phased array devices are used in a wide variety of systems in which it is desired to form or steer a beam of vibratory energy (as used herein, "vibratory energy refers to different modalities of radiating energy through a medium, including, but not limited to, electromagnetic energy (as in radar), sonic energy (as in sonar), and ultrasonic energy (as in ultrasound devices)). In a radar system, for example, the sequential activation of antenna elements is used to direct the beam of electromagnetic energy in a desired direction, that is, along a selected axis with respect to the array of antenna elements. Similar beamsteering can be used in ultrasound devices for medical or industrial purposes and sonar systems.
Conventional phased array beamsteering systems typically have electronic components to generate the sequential time delays to generate the radiated signal and to process the return signal. One disadvantage of such electronic systems is the large number of electrical delay elements and amplifiers that are required. Such electronic beamsteering systems are complex to assemble and maintain, and the number and nature of components present numerous potential failure modes. Further, signal losses in the delay components coupled in sequential stages may reduce the system's operational sensitivity.
For many applications, such as ultrasound devices, it is desirable to have a large number of transducer elements in an array to provide a higher resolution focused beam for tissue diagnostics, as well has high transmit power. Electronic-only beamsteering systems for large arrays (e.g., 512 to 1024 transducer element arrays compared to conventional arrays of 128 to 256 transducer elements) is large and bulky and requires a complex network of EMI sensitive RF networks. Further, long time delays necessary for such a large ultrasound array result in high RF losses and increased timing control sensitivity.
Opto-electronic signal processing has been applied in a variety of communications and radar systems. For example, for phased array radar systems, opto-electronic processing can be used to generate either time delayed or phase shifted optical signals. See, e.g., U.S. Pat. Nos. 5,117,239 and 5,191,339, which are assigned to the assignee herein and are incorporated by reference. For long time delays (e.g. about 4 ns. or more) fiber delay units are used. Such processing systems depend upon use of linearly polarized light, which generally necessitates that system components be selected that minimize the undesired shifting of polarization of the light signals. Most polarization maintaining (PM) fibers, however, not only have a higher cost but also have a high induced optical birefringence that can cause shifts in polarization orientation of the light passing through the fiber from thermal effects or mechanical perturbations.
A phased array beamsteering system is desirably compact, relatively immune to undesirable electromagnetic radiation, and straightforward to fabricate, operate, and maintain. Such a system also preferably has inertialess, motion-free operation that is readily adapted for reliable operation in a number of environments, such as portable equipment, or in aircraft or ships.
It is accordingly an object of this invention to provide a rugged high performance optical beamsteering system in which uncommanded shifts in the polarization orientation of light passing therethrough is minimized.
It is a further object of this invention to provide a high performance optical beamsteering system adapted for use in ultrasound equipment that is relatively compact, lightweight, and inertialess.